This invention relates to an improved apparatus and method for restoring catalytic activity of solid particles previously used to promote chemical conversion processes. More particularly, the invention relates to a method and apparatus for separating, at high temperatures, solid particulate matter, used in promoting hydrocarbon conversions, from a mixture of vapor and solid particulate matter. The invention provides a technique for maintaining both high separation efficiency and low particulate attrition at high loading conditions.
In many instances throughout the chemical and hydrocarbon processing industries, chemical reactions occur which are promoted by relatively small catalyst particles in fluidized bed catalytic reactions (e.g., catalyst diameters ranging from about 10 microns to about 500 microns.) One process used extensively in the petroleum industry which utilizes small catalyst particles is the catalytic cracking of higher boiling hydrocarbons to gasoline and other lower boiling components. The apparatus used for carrying out this chemical conversion (e.g., cracking of a feedstock) or reforming (e.g., hydrocarbon gas oil) includes a reaction zone where the relatively small catalyst particles and feedstock are contacted at chemical conversion (e.g., hydrocarbon cracking or reforming etc.) conditions to form at least one chemical conversion product (e.g., hydrocarbons having a lower boiling point than the hydrocarbon feedstock and/or a higher octane rating.)
Often, while promoting the desired chemical conversion, the catalyst particles have deposited thereon carbonaceous materials such as carbon, coke and the like which act to reduce the catalytic activity of these particles. Apparatus which is used to restore the catalytic activity of such particles often includes a regeneration zone where the deposit-containing solid particles are contacted with oxygen-containing vapor at conditions to combust at least a portion of such deposited material.
Operation of each of the systems referred to above involves the formation of a mixture of solid particles and vapor followed at some point in time with a separation of at least a portion of the solid particles from the vapor-particle mixture. Therefore, both the apparatus for carrying out chemical conversion and the apparatus for restoring the catalytic activity of the solid catalyst particles include at least one separation apparatus wherein the mixture of solid particles and vapor formed in either a reaction of a regeneration zone, respectively, is at least partially separated. Such separation apparatus often involves a conventional cyclone precipitator or separator.
Processing solid catalyst particles through cyclone precipitators may cause the solid catalyst particles to break up and/or form "fines" by attrition. The resulting particle fines are often of such a size that they cannot be effectively separated from the vapor, and are lost from the system. This results in the loss of valuable catalyst and the discharge of potential air pollutants. Accordingly, it is advantageous to provide for a cyclone having low or reduced rates of attrition of the solid catalyst particles.
"Attrition" generally refers to the fraction of solid particles which are converted to less than about 20 microns.sup.1/ in average diameter as a result of one or more collisions between solid particles, alone or in connection with a solid cyclone wall or other surface. A cyclone with "low attrition" is one in which less than about 3.0.times.10.sup.-6 of all catalyst particles are converted to less than about 20 micron size during a separation of particles from vapor therein. FNT 1/ As used herein, a "micron" is equivalent to one micrometer, or 10.sup.-6 meter.
In addition to low attrition, cyclones preferably have a high separation efficiency, e.g., an efficiency in separating from the mixture of solid particles and vapor about 95-99% of the solid particles larger than about 20 microns in diameter. However, conventional cyclone separation art teaches that in scaling a cyclone for high loading conditions (e.g., processing a stream having a high volumetric flow rate "F", typically on the order of about 200-600 cubic feet of fluid per second) either high separation efficiency or low attrition must be sacrificed. Engineers faced with the problem of specifying the dimensions of a cyclone for high loading conditions try for optimum balance in the trade-off between separation efficiency and attrition.
In order to attain both high separation efficiency and low attrition, the prior art has required that smaller sized cyclones should be used. The rationale for this position is based upon a relatively complex relationship between the flow patterns inside the cyclone, the tangential velocity of the particle at the cyclone wall and the effect of collisions between the cyclone wall and the solid particles. For example, in order to provide high separation efficiency, the gas revolution velocity (and thus the motion of the solid particles toward the outer walls) should be high. However, high centrifugal forces create strong friction forces between the solid particles and the cyclone wall, and these strong friction forces coupled with high tangential velocities at the cyclone wall increase the rate of attrition. In order to provide low attrition, the tangential wall velocity should be low. However, lower tangential velocities in conventional cyclones designed for high loading conditions are typically achieved by reducing the velocity of gas revolutions, which, in turn, reduces separation efficiency. Thus arises the trade-off between separation efficiency and attrition. The use of smaller sized cyclones minimizes the disadvantageous trade-off because smaller radius cyclones provide greater centrifugal forces at relatively slower tangential velocities.
The use of smaller sized cyclones leads to further complications, particularly in the high volume applications which characterize many industries and the petroleum industry in particular. For example, the smaller sized cyclones have a smaller operating capacity. In order to process a product stream of a given size, two or more smaller sized cyclones must be used in the place of one larger sized cyclone. The smaller sized cyclones in such situation are typically set up in parallel operation with the product stream divided between them, resulting in a more complex and more expensive design. Importantly, the use of multiple, smaller cyclones results in a more space consuming design than a single larger cyclone.
In some situations, the option of using several smaller size cyclones may not be available as a practical matter because the necessary amount of space does not exist. Inadequate space is especially likely to present a problem where new cyclones are being installed to upgrade an existing facility; the available space in the old facility may not adequately accommodate several, smaller new cyclones. Frequently, even in new chemical processing plants, the amount of space provided for restoration of the catalytic particles is inadequate to effectively allow using the smaller sized units, assuming that such a choice were otherwise practicable.
Heretofore, no adequate alternative choice for providing a high capacity, high efficiency, low attrition cyclone or restoration method, particularly one operable within commonly available space limitations, has been available. The present invention provides a solution to this problem.